1. Field of the Invention
The present invention relates to a semiconductor device typified by a thin film transistor and to a fabrication method thereof. The present invention also relates to a semiconductor device using a crystal silicon thin film formed on a substrate such as a glass substrate and quartz substrate and to a fabrication method thereof. Further, the present invention relates to an insulated gate type semiconductor device such as a thin film transistor and to a fabrication method thereof.
2. Description of Related Art
Hitherto, there has been known a thin film transistor using a silicon film, i.e. a technology for forming the thin film transistor by using the silicon film formed on a glass substrate or quartz substrate. The glass substrate or quartz substrate is used as the substrate because the thin film transistor is used for an active matrix liquid crystal display. While a thin film transistor has been formed by using an amorphous silicon film (a-Si) in the past, it is being tried to fabricate the thin film transistor by utilizing a silicon film having a crystallinity (referred to as “crystal silicon film” hereinbelow in the present specification as necessary) in order to enhance its performance.
The thin film transistor using the crystal silicon film allows to operate at a high speed by more than two digits as compared to one using the amorphous silicon film. Accordingly, while peripheral driving circuits of an active matrix liquid crystal display have been composed of external IC circuits, the crystal silicon film allows them to be built on the glass substrate or quartz substrate similarly to the active matrix circuit. Such structure is very advantageous in miniaturizing the whole apparatus and in simplifying the fabrication process, thus leading to the reduction of the fabrication cost.
Hitherto, a crystal silicon film has been obtained by forming an amorphous silicon film by means of plasma CVD or low pressure thermal CVD and then by crystallizing it by implementing a heat treatment or by irradiating laser light. However, it has been the fact that it is difficult to obtain a required crystallinity across the wide area through the heat treatment because it may cause nonuniformity in the crystallization. Further, although it is possible to obtain the high crystallinity partly by irradiating laser light, it is difficult to obtain a good annealing effect across the wide area. In this case, the irradiation of the laser light is apt to become unstable under the condition for obtaining specifically a good crystallinity.
By the way, the inventors et. al. have developed a technology for obtaining the crystal silicon film through a heat treatment at a lower temperature than that of the prior art by introducing a metal element (e.g. nickel) which promotes the crystallization of silicon to the amorphous silicon film (Japanese Patent Laid-Open Nos. Hei. 6-232059 and Hei. 7-321339). These methods allow not only the crystallization speed to be increased and the crystallization to be achieved in a shorter time, but also a high crystallinity to be obtained uniformly across the wide area, thus having a crystallinity which fits for practical use, as compared to the prior art crystallization of amorphous silicon film implemented only by way of heating or by way of the irradiation of laser light.
However, because the metal element is contained within or on the surface of the crystal silicon film, the amount thereof to be introduced has to be controlled very carefully, thus posing a problem in its reproducibility and stability (electrical stability of a device obtained). Specifically, there is a problem that an elapsed change of the characteristics of a semiconductor device to be obtained is large or an OFF value, in case of a thin film transistor, is large, due to the influence of the remaining metal element. That is, although the metal element which promotes the crystallization of silicon plays the valuable and useful role in obtaining the crystal silicon film, its existence becomes a minus factor which causes various problems after obtaining the crystal silicon film once.
Then, after conducting a large number of experiments and discussions from various aspects in order to solve the problem in forming the crystal silicon film by introducing the metal element (e.g. nickel) which promotes the crystallization of silicon to the amorphous silicon film and by treating by heat as described above, the inventors et. al. have found that the metal element contained and remaining in the crystal silicon film may be eliminated or reduced by the specific method described later, thus reaching to the present invention.
By the way, because an active matrix liquid crystal display is small, is light and is able to display fine motion pictures at high speed, it is being expected to become the mainstream of displays of the future. However, because it has a limit that a substrate composing the liquid crystal display needs to be translucent, its type is limited. A glass substrate, a quartz substrate or a plastic substrate may be cited as an example thereof.
However, among them, the plastic substrate has a problem that it lacks in heat resistance and the quartz substrate has a problem that it is very expensive and its cost is more than 10 times of the glass substrate especially when it is widened, thus lacking in cost performance, though it can withstand a high temperature of about 1000° C. or 1100° C. Accordingly, the glass substrate is widely used in general from the reasons of heat resistance and economy.
Currently, the performance required for the liquid crystal displays is getting higher and higher and the performance and characteristics required for a thin film transistor (hereinafter referred to as a TFT as necessary) used as a switching element of the liquid crystal displays is also getting higher. Due to that, while the research and development for forming the crystal silicon film having the crystallinity on the glass substrate are being actively conducted, the crystal silicon film is formed on the glass substrate by adopting the method of forming the amorphous silicon film and of crystallizing it by treating by heat or by irradiating laser light at the present.
That is, because the heat resistant temperature of the glass substrate is normally about 600° C., though it depends on a type thereof, a process which exceeds the heat resistant temperature of the glass substrate cannot be adopted in the step for forming the crystal silicon film. Therefore, a method for forming the amorphous silicon film by means of plasma CVD or low pressure CVD and crystallizing it by heating at a temperature below that heat resistant temperature has been adopted in forming the crystal silicon film on the glass substrate. The method of crystallizing the silicon film by irradiating laser light also allows the crystal silicon film having an excellent crystallinity to be formed on the glass substrate and has an advantage that the laser light will not damage the glass substrate thermally.
However, the crystal silicon film crystallized from the amorphous silicon film by the above-mentioned technologies has had a large number of defects caused by dangling bond and the like. Because these defects are the factor of degrading characteristics of the TFT, it is necessary to passivate the defects at the interface between an active layer and a gate insulating film and the defects within and at the boundary of the crystal grains of the silicon of the active layer in fabricating the TFT by utilizing such crystal silicon film. The defects at the grain boundary in particular are the greatest factor of scattering charge, but it is very difficult to passivate the defects at the grain boundary.
Meanwhile, it is possible to compensate the defects at the grain boundary of the crystal silicon film by Si in fabricating a TFT on the quartz substrate because it is possible to implement a heat treatment at a high temperature of about 1000° C. or 1100° C. for example. In contrary to that, it is difficult to implement the heat treatment in high temperatures in fabricating the TFT on the glass substrate, so that the defects of the grain boundary of the crystal silicon film are passivated by hydrogen by implementing a hydrogen plasma treatment in an atmosphere of about 300 to 400° C. normally in the final stage of the process.
An n-channel type TFT presents a practical field-effect mobility by implementing the hydrogen plasma treatment. On the other hand, the effect of the hydrogen plasma treatment is not so remarkable in a p-channel type TFT. It is construed to happen because a level caused by the defect of the crystal is formed in a relatively shallow domain under a conduction electron zone. Although it is possible to compensate the defect of the grain boundary of the crystal silicon film by implementing the hydrogen plasma treatment, an elapsed reliability of the TFT or the n-channel type TFT in particular which has been treated by the hydrogen plasma is not stable because the hydrogen compensating the defect is apt to be desorbed. For instance, if the n-channel type TFT is energized for 48 hours in an atmosphere of 90° C., its mobility is reduced to a half.
Further, although the quality of the crystal silicon film obtained by irradiating laser light is good, ridges (irregularity) are formed on the surface of the crystal silicon film if the thickness of the film is less than 1000 angstrom. When laser light is irradiated to the silicon film, the silicon film is melt instantly and expands locally. The ridges are formed on the surface of the crystal silicon film to relax internal stress caused by this expansion. A difference of elevation of this ridge is about ½ to 1 time of the thickness of the film. For instance, when laser annealing is implemented after crystallizing an amorphous silicon film whose thickness is about 700 angstrom by way of heating, ridges of 100 to 300 angstrom in height are formed on the surface thereof.
Because a potential barrier and a trap level caused by the dangling bond, distortion of lattice and the like are formed at the ridges on the surface of the crystal silicon film in an insulated gate type semiconductor device, the level of the interface between the active layer and the gate insulating film becomes high. Further, because the peak of the ridge is sharp and thus an electric field is apt to concentrate there, it may become a source of leak current, causing dielectric breakdown in the end. Further, because the ridge on the surface of the crystal silicon film damages a coating quality of the gate insulating film deposited by way of sputtering or CVD, it degrades the reliability of insulation by causing defective insulation.